1. Field of the Invention
The present invention relates to an organic electroluminescent display device and a method of producing the same, and more particularly, to an organic electroluminescent display device having greatly improved coupling efficiency of light generated by an organic layer.
2. Description of the Related Art
Typically, organic electroluminescent display devices are self-emissive display devices that emit light by electrically exciting a fluorescent organic compound. Organic electroluminescent display devices have advantages over liquid crystal displays (LCDs) because they require a low driving voltage, have easy thin film formation properties, provide a wide viewing angle, and have fast response speeds. Therefore, there has been increasing interest in organic electroluminescent display devices as next generation displays.
Such an organic electroluminescent display device was developed as a layered-type display device by Eastman Kodak and commercialized as a green display device with an increased lifespan by Pioneer. Novel organic materials having various molecular structures have been developed and the development of color displays having beneficial properties such as low direct current driving voltage, reduced thickness and self-emission is actively taking place.
Generally, an organic electroluminescent display device is produced by forming a patterned organic layer on a transparent insulating substrate such as glass and forming electrode layers on upper and lower surfaces of the organic layer. The organic layer is composed of copper phthalocyanine (CuPc), N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) or tris-8-hydroxyquinoline aluminum.
The principle of image formation in an organic electroluminescent display device is as follows. When anode and cathode voltages are applied to electrodes, holes injected from the electrode to which the anode voltage is applied migrate to an emission layer via a hole transport layer and electrons are injected from the electrode to which the cathode voltage is applied to the emission layer via an electron transport layer. In the emission layer, the electrons and the holes are recombined to generate excitons. When the excitons are changed from an excited state to a ground state, fluorescent molecules of the emission layer emit light, thereby forming an image.
Light efficiency of the organic electroluminescent display device as described above is divided into internal efficiency and external efficiency. The internal efficiency depends on a photovoltaic conversion efficiency of an organic light emitting material and the external efficiency depends on a refractive index of each layer in the device. The external efficiency is also known as light coupling efficiency. An organic electroluminescent display device has a lower light coupling efficiency than other display devices such as CRT, PDP and FED. Thus, there is room for improvement in characteristics such as luminance, and lifespan of the organic electroluminescent display device.
FIG. 1 schematically illustrates a problem of reduction in light coupling efficiency of a conventional organic electroluminescent display device. Referring to FIG. 1 (Lu et al., APL 78 (13), p. 1927, 2001), in the conventional organic electroluminescent display device, light generated by an organic layer undergoes total internal reflection at an ITO/glass interface and a glass/air interface. In a conventional organic electroluminescent display device, the light coupling efficiency is only about 23% and the remaining light dissipates without being emitted outside.
Various methods for improving the light coupling efficiency of an organic electroluminescent display device having been proposed and, recently, methods related to introduction of a diffraction grating are studied and reported.
Japanese Patent Laid-Open Publication No. Hei 11-283751 discloses an organic electroluminescent display device including one or more organic layers interposed between an anode and a cathode, and a diffraction grating or a zone plate.
Such an organic electroluminescent display device is difficult to produce since a separate diffraction grating should be mounted on the surface of a substrate or a finely patterned electrode layer. Thus, an improvement in productivity cannot be achieved. Furthermore, when an organic layer is formed on the uneven surface of the diffraction grating, the surface roughness of the organic layer increases, resulting in a reduction in durability and reliability of the organic electroluminescent display device.
Korean Patent Laid-Open Publication No. 2003-0070985 discloses an organic electroluminescent display device having an optical loss preventing layer having regions with different refractive indices inserted between two layers with high refractive indices selected among a first electrode layer, an organic layer and a second electrode layer. Referring to FIG. 2, the diffraction grating is formed on a substrate to diffract the guided light, thereby making a diffraction angle less than the critical angle of total internal reflection so that light is emitted.
However, in such a organic electroluminescent display device, when diffused reflection is caused due to voids and unevenness generated in the formation of the diffraction grating, the quantity of light emitted to a front surface (external direction) decreases, resulting in a reduction in luminous efficiency. When a high refractive material is deposited or coated on the diffraction grating, the interface between the high refractive layer and the first electrode becomes uneven, and thus the first electrode also becomes uneven, resulting in a reduction in external light coupling efficiency.
In organic electroluminescent display devices according to the conventional art, light distribution is concentrated on the first electrode, and thus an increase in the luminous efficiency due to the diffraction grating is relatively low.